1. Field of the Invention
This invention relates to a catalyst deterioration-determining system for detecting deterioration of a catalyst arranged in an exhaust passage of the internal combustion engine for purifying noxious components contained in exhaust gases emitted from the engine.
2. Prior Art
A technique for detecting deterioration of a catalyst arranged in an exhaust passage of an internal combustion engine for purifying noxious components contained in exhaust gases emitted from the engine has already been proposed by the present assignee in Japanese Laid-Open Patent Publication (Kokai) No. 6-212955, in which while the feedback control of the air-fuel ratio of a mixture supplied to the engine is executed in response to the output from an oxygen concentration sensor arranged in the exhaust passage at a location downstream of the catalyst (hereinafter referred to as "the downstream oxygen concentration sensor"), the inversion period of the output from the downstream oxygen concentration sensor occurring during the feedback control is utilized in detecting deterioration of the catalyst.
However, this conventional technique is based on the assumption that the engine is in a steady operating condition. Accordingly, when the air-fuel ratio of the mixture supplied to the engine fluctuates due to changes in the operating condition of the engine, it results in variations in the measured inversion period of the output from the downstream oxygen concentration sensor, which impedes accurate determination of the catalyst deterioration.
Further, the above-mentioned publication also discloses "a quick OK determination method" in which once a measured time period between inversions of the output from the downstream oxygen concentration sensor is longer than a predetermined time period, it is quickly determined that the catalyst is normal, followed by terminating the monitoring for catalyst deterioration.
However, this quick OK determination method is also based on the assumption that the engine is in a steady operating condition, so that when the air-fuel ratio of the mixture supplied to the engine fluctuates, there can be made an erroneous determination that the catalyst is normal, even when it is actually deteriorated. For example, in the case of an engine equipped with an evaporative emission control system for purging temporarily-stored or adsorbed evaporative fuel to the intake system when the engine is in a predetermined operating condition, when the purging of evaporative fuel is executed during deceleration of the engine, the output from the downstream oxygen concentration sensor can continuously indicate that the air-fuel ratio is rich for a relatively long time period, i.e. it can remain uninverted for a long time period. In such an event, there can be made an erroneous determination that the catalyst is normal, even when the catalyst is actually deteriorated.